SOLIDIFICATION OF METALS

Cooling curve for a pure metal during casting A pure metal solidifies at a constant temperature equal to its freezing point(same as melting point)

CHVORINOV’S RULE TST=Cm(V/A)^n Where, TST=total solidification time V=Volume of the casting A=surface area of casting n=exponent usually taken as 2 Cm=constant which depends upon mould material.

What CHVORINOV’S RULE tells us? A casting with a higher modulus( volume to surface area ratio)cools and solidifies more slowly than the one with a lower modulus. To feed molten metal to the casting, TST of the riser must be greater than TST of the casting. Since mould constants of riser and casting will be equal, design the riser to have a larger modulus so that the main casting solidifies first.

Requirement of the riser to feed the casting M R = 1.2 X Mc Where, M R = MODULUS OF RISER Mc = MODULUS OF CASTING

CASTING DEFECTS

CASTING DEFECTS: Defects may occur due to following reasons: Fault in design of casting pattern Fault in design on mold and core Fault in design of gating system and riser Improper choice of molding sand Improper metal composition Inadequate melting temperature and rate of pouring

SURFACE DEFECTS: Blow, Scar, Blister, Drop, Scab, Penetration Drop

INTERNAL DEFECTS: Blow holes, Porosity, Pin holes, Inclusions, Dross  Trapped gases and dirty metal.  Hard ramming or improper venting.  Excessive moisture

DEFECTS due to POURING OF THE MELT MISRUN,COLD SHUT, SLAG OR DROSS INCLUSION